As is well known, a filter is placed in the exhaust system of diesel engines to remove soot from the exhaust gases of the engine. The filter must be changed or cleaned from time to time to ensure that soot accumulations do not adversely affect engine operation. It is known to remove or incinerate the soot particles by subjecting the filter, in situ, to heat from a fuel burner or other heat generating device, or from suitable running of the engine. Incineration is to be performed when the accumulation has reached a level where further accumulation would adversely affect engine performance or before that incineration would produce excessive temperatures and possibly damage the filter. There is a need, therefore, for a method and apparatus which monitors the level of soot accumulation and provides a signal when the accumulation reaches a predetermined level.
Soot accumulations exhibit dielectric properties. Accordingly, it is possible to monitor the level of soot accumulation on a diesel engine filter medium by detecting changes in the effective dielectric properties of the filter medium. The complex permittivity of a material is comprised of two components: a real component called the "dielectric constant" and an imaginary component called the "dielectric loss factor". Changes in either of these components can be detected using RF interrogation methods. It should be mentioned at this point that the dielectric constant and loss factor of soot increases with increasing temperature. This affects both transmission and reflection (resonance) type of measurements.
One method of applying this concept to monitoring soot levels in diesel filters is to construct the filter housing or containment in the form of a RF waveguide and then periodically excite the waveguide with RF energy at a fixed frequency and measure the reflected power. The reflected power will be a function of soot accumulation on the filter. More specifically, for any RF system, it is usually possible to determine a frequency at which the electrical load, i.e. the filter medium, the diesel soot and the filter containment, represents a matched impedance with respect to the power source. In other words, the equivalent electrical resistance, capacitance and/or inductance of the load are matched to the RF power source. When the load impedance is perfectly matched to the power source, all emitted RF power is absorbed by the load. If the impedance is not matched to the RF source, some of the RF power will be reflected from the load. The degree of load mismatch determines the mount of reflected power and, hence, reflected power can be used to measure the change in the effective dielectric constant. This method can be generally referred to as a reflectance or resonance type of measurement.
U. S. Pat. No. 4,477,771 granted to the General Motors Corporation on Oct. 16, 1984 describes a method of detecting soot content in a particulate trap using this method. The method detects changes in the effective dielectric constant only. The patent provides a metal filter housing constructed as cylindrical waveguide which defines a closed, RF resonance cavity for receiving a ceramic filter. A single probe is positioned at one end of the cavity and behaves as both a transmitting and a receiving antenna. A reflective screen is positioned at the opposite end of the cavity. All connecting exhaust pipe diameters are below the cutoff diameter of a circular waveguide needed to transmit the RF energy at the frequencies used in the device. The probe is connected to a RF source through a directional coupler and an isolator. A detector is also connected to the probe through the directional coupler. In one mode of operation of the device, the RF source is operated at the resonant frequency of the cavity when the filter is loaded with particulates to its maximum desired accumulation and the detector is operated to detect a null condition in the reflected signal which occurs at the resonant condition. Upon detecting such a condition, the detector generates an output signal operable to effect operation of a lamp or alarm. In a second embodiment, the reflective screen is replaced by a second probe positioned at the remote end of the cavity. One probe is connected to the power source and the other probe is connected to the detector.
There are a number of practical and technical problems with this approach. From a practical point of view, it is important to understand that the commercial viability of a RF-based device depends on its component count and, more on its component price. In this latter respect, higher operating frequencies incur higher component and fabrication costs. The device also tends to display poor sensitivity and is prone to large measurement errors due to the effect of temperature on the effective dielectric constant for reasons described below.
From a technical point of view, there are two factors which must be considered and which have been overlooked by the prior art. One factor relates to the properties of the filter containment or housing and the other relates to the properties of soot. Dealing firstly with the filter housing, based on wavelength considerations, there is a frequency below which a waveguide will not allow RF energy to propagate without significant attenuation. The frequency below which this occurs is called the "cutoff frequency" for that waveguide geometry. The formula for calculating this cutoff frequency and the attenuation for the transmission of frequencies below cutoff is well known to those knowledgeable in the art. It can be shown, for example, that the cutoff frequency for a 14.4 cm diameter filter is greater than 1.2 GHz and for a 30.5 cm diameter filter, the cutoff frequency is greater than 0.5 GHz. If the filter containment is a cylindrical resonator and the frequency for the lowest mode (and frequency) for resonance is calculated, one finds that for the smaller filter (14.4 cm Diameter.times.15.24 cm Long) the TE.sub. 111 resonant frequency is 1.6 GHz and for a TM.sub.111 resonance, the resonant frequency is 1.9 GHz. Similarly, for a 28.6 cm Diameter.times.30.48 cm long filter, the TE.sub.111 resonant frequency is 0.79 GHz and, for the TM.sub.111 mode, the frequency is 0.94 GHz. These calculations clearly indicate that conditions for resonance require even higher frequencies than for transmission. These high frequencies result in high component and fabrication costs.
The properties of soot (carbon particulates) also have a significant impact on viability of RF-based measurement methods. Soot is a particularly lossy dielectric and it is for this reason that carbon black (soot) is added to materials like rubber to increase the carrier's ability to be heated in a RF field. Terminal loads for RF systems are also constructed of carbon. The dielectric constant of the soot changes with temperature and hence the effective dielectric constant of the filter changes with temperature. This means that the resonant frequency shifts with changes in both soot accumulation and temperature. Clearly, this effect must be accounted for in measuring soot accumulation in a filter heated by hot diesel exhaust. This factor adds to the complexity and cost of the device.
For either of the methods proposed in the above described patent, the metal housing containing the empty filter must act as a narrow bandpass RF filter in order to make the measurements described in the patent. That is to say, the resonant cavity thus formed should allow energy to enter the cavity over only a very narrow range on either side of the resonant frequency and reject or reflect RF energy at all other frequencies (i.e., a narrow bandpass RF filter). Unfortunately for the methods described in the patent, the accumulation of soot not only changes the effective dielectric constant of the filter, thereby shifting the resonant frequency of the cavity, but it is also causes the cavity to increase its bandpass frequency range due to the effects of the very high dielectric loss factor of the soot, a factor not considered in the patent. In fact, above a range of soot load and temperature combination, the soot becomes a purely resistive load over a wide range of frequencies (i.e., it becomes a broadband terminal carbon load). When the load becomes mainly resistive in nature, reflections drops virtually to zero. Since this phenomenon is broadband in nature, it is no longer possible to measure a resonant frequency (i.e., there is no difference in the amount of power being reflected over a wide range of frequencies).
In summary, reflectance or resonance type measurements of the type described in the above mentioned patent arc precluded from using frequencies below the resonant frequency defined by the geometry of each filter and/or its metal containment. There is a manufacturing cost penalty associated with the relatively high frequencies that must be used by these methods. The high loss factor of the soot, as determined by soot concentration, temperature and RF frequency, places severe restrictions on the range of soot concentration that can be measured. In short, these methods are not commercially viable.